The present invention relates to novel filled polypropylene compositions which are able to pass a more stringent flame retardance test than ASTM D-635.
In the past "flame retardant" polypropylene compositions, i.e. those passing ASTM test D-635 have been prepared by incorporation of chlorinated compounds into polypropylene resins. Additional flame retardants such as antimony oxide are often present in these compositions for the purpose of further enhancing flame retardancy.
One group of chlorinated compounds that have been suggested for such use, are the chlorinated paraffin hydrocarbons containing between about 40 to 80% chlorine and averaging 18 to 36 carbon atoms. Although the chlorinated paraffin hydrocarbons can be added in small amounts to polypropylene without any particular operating problems, it has been found that when added in quantities that should be sufficient for imparting acceptable flame retardance according to present standards the chlorinated hydrocarbons are incompatible with the polypropylene and will leach out during processing. The resultant tacky product will not pass a flame retardance test such as UL 94(1/16) with V-1 rating and will drip.
Another group of chlorinated compounds that is used as flame retardants in polypropylene compositions are the chlorinated polyethylenes containing between about 20 to 55 percent bound chlorine. However, even when added to polypropylene in a 50:50 weight ratio together with antimony oxide it was found that the composition will not pass the aforementioned UL 94 test and will burn.
In adding various inorganic filler such as glass, asbestos etc. to polypropylene containing any one of the aforementioned chlorinated compounds, flame retardance is somewhat improved due to the dilution effect, however not to an acceptable level.
Recently, compounds have been developed that act as combined fillers and flame retardants for various resins. A characteristic common to these compounds is that they contain bound moisture, which upon exposure of the filled resin to flame is released as water vapor. Thus, these fillers act as a flame-inhibiting heat sink. One such filler-flame retardant of particular interest is alumina trihydrate, which is effective in various thermoset resins, e.g. polyester, epoxies, phenolics and the like. However, due to the relatively low temperature at which water is liberated (230.degree. C), the use of alumina trihydrate is limited to resins which can be processed, i.e. molded or cast, at temperatures well below the decomposition temperature of the alumina trihydrate. This limitation has precluded the use of alumina trihydrate in polypropylene compositions, which are usually processed at higher temperatures.
It is therefore an object of this invention to provide a filled polypropylene composition having improved flame retardance properties and which can be processed without decomposition of the filler.